From the past, an optical measurement method is known in which a slab-type optical waveguide is employed, and wherein only a labeled fluorescent substance, existing in the vicinity of a surface of the slab-type optical waveguide, is excited by an evanescent wave component scarcely penetrating from the slab-type optical waveguide, and existence or non-existence of immunoreaction, or degree of immunoreaction, is measured based upon the excited fluorescent light. To actualize the method, an apparatus which is shown in FIG. 10 is proposed (refer to Swiss Patent Specification No. 2799/85-2, and an official gazette of Tokukaisho No. 63-273040). In the apparatus, a reaction vessel 92 is formed in one body on one face of a slab-type optical waveguide 91 which has a prism at an edge portion for light introduction. An exciting light sent from a laser light source or the like, which is not shown, is introduced in the slab-type optical waveguide 91 through a dichroic mirror 93. Fluorescent light radiated from labeled fluorescent substance is sent through the slab-type optical waveguide 11, and the outgone fluorescent light is separated from an exciting light path by the dichroic mirror 93, and the outgone fluorescent light is incident on an optical detector 95 through an optical filter 94.
When the above-mentioned arrangement is employed, for example, antibodies 96 are previously immobilized to the surface of the slab-type optical waveguide 91, antigens 97 in a test liquid are received by the antibodies 96, fluorescent labeled antibodies 98, which are formed by labeling antibodies with fluorescent substance, are further received by the received antigens 97. That is, the quantity of the received fluorescent labeled antibodies 98 is determined based upon the quantity of the antigens 97 in the test liquid. And, only the label fluorescent substance 98a of the received fluorescent labeled antibodies 98 are excited so as to generate fluorescent light by the evanescent wave component which is obtained by introducing an exciting light in the slab-type optical waveguide 91. Therefore, the radiated fluorescent light intensity is in proportion to the quantity of the antigens 97 in the test liquid. The fluorescent light is guided in the slab-type optical waveguide 91.
Consequently the existence or non-existence of an immunoreaction, or degree of immunoreaction, is measured by reflecting only fluorescent light guided through the slab-type optical waveguide 91 by the dichroic mirror 93 and which is incident on the optical detector 95 following screening out the noise light component with the optical filter 94.
But, a part of the exciting-light becomes leaking light so as to excite labeled fluorescent substance 98a even at a position sufficiently apart from the surface of the slab-type optical waveguide 91, because the slab-type optical waveguide 91 has surface irregularity (surface roughness, disfigurement) and the like.
Meanwhile, fluorescent light which is radiated from the labeled fluorescent substance 98a sufficiently apart from the surface of the slab-type optical waveguide 91, is not introduced in the slab-type optical waveguide 91 by a angle more than an critical angle. But, when the distance between the labeled fluorescent substance 98a and the surface of the slab-type optical waveguide 91 becomes nearly equal to the wavelength, the fluorescent light component of which the introduction angle is more than the critical angle increases. Therefore, the fluorescent light guided in the slab-type optical waveguide 91 has sufficient non-sensitivity to fluorescent light other than the fluorescent light radiated in the vicinity of the surface of the slab-type optical waveguide 91.
But, though the fluorescent substance 98a, excited at a position sufficiently apart from the surface of the slab-type optical waveguide 91, radiates non-directional fluorescent light, a part of the fluorescent light is incident on the optical detector 95 through an edge portion of the prism section, side walls of the reaction vessel 92 and the like. More particularly, when the field of view for detection of the optical detector 95 is positioned so as to receive only the fluorescent light guided in and outgone from the slab-type optical waveguide 91, the fluorescent light that passes through the edge portion of the prism section, the side walls of the reaction vessel 92 and the like is scarcely received by the optical detector 95. Such positioning with high accuracy is actually difficult. Even when such positioning with high accuracy is performed, dislocation of the determined positioning caused by vibration and the like of the apparatus should be taken into consideration. Thus, the field of view for detection of the optical detector 95 should be determined to be wide, and the fluorescent light that passes through the edge portion of the prism section, side walls of the reaction vessel 92 and the like is received by the optical detector 95. And, the fluorescent light is not influenced by the optical filter 94, and is received by the optical detector 95 as it is. Therefore the measurement accuracy is lowered.
In the foregoing, description was made only in the case that optical measurement (fluorescent immunity measurement) in the vicinity of the surface of the slab-type optical waveguide 91 is carried out based upon fluorescent light. When optical measurement in the vicinity of the surface of the slab-type optical waveguide is carried out based upon light other than fluorescent light, for example, phosphorescence, the similar disadvantages arise.